1. Technical Field
The present invention is directed to imaging sensors and associated imaging devices. In particular, the invention is directed towards a more efficient image sensor having enhanced blue response and cross-talk suppression based on properly positioned and doped wires of a semiconductor substrate.
2. Related Art
Charge Coupled Device (CCD) technologies have always customized the fabrication process to properly position the junctions and depletion depths for optimal spectral sensitivities and minimum cross-talk. In the course of using standard CMOS technologies to build image sensors, attention needs to be paid to the location depth of photodiode junctions.
The depth of the depletion of the photodiode is also important as well. This, in combination with the depth of the photodiode junctions, determine the spectral sensitivity and optical cross-talk of an imager.
Standard CMOS technology indicates the edge of the depletion region, meaning the junction depth plus the depletion depth of a source/drain diode at VDD reversed bias, ranges from 0.25 micron to 0.8 micron.
Comparing these depths with the penetration depth of visible light in silicon, it is apparent that for standard CMOS imagers that most photo carriers are generated in the neutral region. Thus, photo carriers cannot be efficiently collected for the imaging process. Further, this allows for the possibility of excessive cross-talk.
In standard CMOS imagers, most photo carriers for blue light, however, are generated shallower in the substrate. This shallow generation of blue light photo carriers has the problem of surface recombination. Thus, the blue response in a standard CMOS imager is attenuated by this characteristic.
As the CMOS technology is scaled down, this non-optimal carrier collection situation gets worse. As such, present photodiode structures do not allow for enhanced blue response and do not allow for cross-talk suppression between image sensors.
Previous solutions employed standard CMOS N+xe2x88x92Pxe2x88x92 well or P+xe2x88x92Nxe2x88x92 well photodiode structures. These standard CMOS photodiode structures provide shallow junctions. As such these standard CMOS photodiode structures tend to have a low blue response and generate a potential cross-talk problem.
The blue color response in the standard CMOS photodiode structures compares relatively low to green and red color output CMOS photodiode structures. This is primarily due to the loss of photo generated carriers near the diode surface due to surface recombination.
Signal cross-talk in standard CMOS photodiode structures is also a problem due to the standard structure of these semiconductor devices. The shallow depletion region allows carrier diffusion to adjacent pixels, allowing for poor cross-talk responses in standard CMOS photodiode structures.
Many other problems and disadvantages of the prior art will become apparent to those schooled in the art after comparing such prior art with the present invention described herein.
In short, the invention is a light sensor on a die. The light sensor is made of a photodiode layer, a substrate layer, and a carrier direction layer. The photodiode layer is made of a semiconductor material having a charge. The substrate layer is disposed on one side of the photodiode layer, and is made of a semiconductor material of an opposite charge than that of the photodiode layer.
A carrier direction layer is disposed between the surface of the die and the other side of the photodiode layer, opposite the substrate layer. The carrier direction layer is made of a semiconductor material. The material of the carrier direction layer and that of the photodiode layer produces an electric field between the photodiode layer and the carrier direction layer. In this manner photogenerated carriers produced in the photodiode layer or, in the charge collection layer are directed to the photodiode layer by the electric field.
In one embodiment of the invention, the substrate layer is made of P-type semiconductor material. Thus, the photodiode would be made of an N-type semiconductor material.
In another embodiment, the carrier direction layer is made of P-type semiconductor material. In this case, the P-type carrier direction layer and the N-type photodiode layer creates an electric field in which electron carriers are directed to the photodiode layer.
In another embodiment, the carrier direction layer is made of N-type semiconductor material. In this case the carrier direction layer is made of a heavily doped (N+) semiconductor material while the photodiode would be made of a lighter doped (Nxe2x88x92) semiconductor material. The resulting potential would direct holes to the photodiode layer.
In another embodiment, the substrate layer is made of a first layer and a second layer. The first and second layers produce an electric field directing carriers to the photodiode layer. In this case, the first layer could be made of a lightly doped P-type (Pxe2x88x92) semiconductor material, and the second layer is made of a more heavily doped P-type (P+) semiconductor material. The first layer is disposed between the photodiode layer and the second layer.
In another embodiment, the light sensor contains a photodiode layer, a substrate layer, and a carrier direction layer, as described above. In this embodiment, the electric field created between the photodiode layer and the carrier direction layer serves to inhibit surface recombination of photogenerated carriers.
In yet another embodiment, the light sensor contains a photodiode layer, a substrate layer, and a carrier direction layer, as described above. The substrate layer is made of two layers of different doping densities. The substrate layers create a deep electric field that serves to inhibit cross-talk of carriers.
In another embodiment, the invention is a light imager having a plurality of light sensors and control circuitry. The control circuitry controls the output of the plurality of light sensors.
The plurality of light sensors are made of a photodiode layer, a substrate layer, and a charge direction layer, as described previously. The plurality of light sensors may take all the forms of the above mentioned embodiments of the light sensor.